I really hope that this will be done in a way that can be turned off (i.e - display as normal 2D) as this could really be a bad move for the iPad. The advantage of the iPad is that you can just use it casually, on your lap, on an armrest and so-on; but with 3D the angle has to be just-so or the effect doesn't work.
If it were a feature you can turn on and off easily, and maybe turns on automatically when you place the iPad on a stand, then it could be do-able, otherwise it's just going to be a nightmare as unlike handheld gaming devices you don't hold an iPad in any one-way.
In fact, on that point, is it even possible to have a 3D screen that works both horizontally and vertically? I suppose it could be but it might mean generating four separate images and building the screen to separate them, that's quite a bit of wasted GPU power right there, not to mention manufacturing cost on the screen.